Filament support structure having antibowing means



United States Patent Olhce 3,539,858 Patented Nov. 10, 1970 3,539,858 FILAMENT SUPPORT STRUCTURE HAVING ANTIBOWING MEANS Jacob A. Randmer, Wilton, Conn., and George J. Agule,

Sr., Sandgate, Vt., assignors to The Maclllett Laboratories, Incorporated, Springdale, Conn., a corporation of Connecticut Filed Jan. 30, 1968, Ser. No. 701,759

Int. Cl. HOld 1/94, 19/48 US. Cl. 3l3-278 7 Claims ABSTRACT OF THE DISCLOSURE A filament support structure for an electron tube, comprising thin metallic arms which have one end insulatingly attached to a center mast of the tube and which extend radially from opposite sides of the center mast in planes perpendicular to a linear array of parallel filament wires disposed longitudinally on opposite sides of the center mast. The respective filament wires are held in axial alignment by respective apertures located in the center of respective intersecting slots which are accurately spaced apart in the respective metallic sup porting arms. Resilient tangs, formed in the thin metallic arms where the respective slots merge with the respective apertures, are flexed by the respective filament wires of larger diameter passing through the adjacent aperture. The contacting edges of the respective tangs press against the sides of the respective filament wires and sink into the softened material of the wires when the filaments are heated to incandescence. Pressure is maintained on the contacted surfaces of the respective filament wires by the respective tangs during longitudinal movement of the filament wires. The flexibility of the respective thin metallic arms allows the respective filament wires to move longitudinally when the latter are undergoing thermal elongation and permits differences in longitudinal expansion of the respective filament wires.

BACKGROUND OF THE INVENTION This invention relates to an electrode structure for electron discharge tubes and is concerned more particularly with a means for maintaining the alignment of long filament wires between the supported ends of a filament array.

In order to supply the heavy electron emission required for power tube applications, some types of power tubes utilize a linear array of parallel filament wires disposed longitudinally in a central axial plane of the tube. When the linear array of filament wires is located between spaced, parallel sections of the grid and more widely spaced, parallel sections of the anode, both longitudinal halves of the filament array are exposed to the anode and are influenced by a potential on the control grid. A high value of current flows through the filament wires which heats them to an incandescent state and produces electron emission. When heating up to this condition, the filament wires undergo thermal expansion and, as a consequence, elongate in the longitudinal direction. Therefore, to avoid distortion of the filament array, the parallel filament wires are supported in a manner that permits the heated filament strands to expand longitudinally during operation of the tube.

In prior art tubes of the described type, supporting members are provided at both ends of the longitudinal filament array to hold the filament wires in axial alignment. Usually, resilient members are attached to one end of the respective filament wires to compensate for thermal elongation and contraction of the respective filament strands during heating and cooling of the tube. However, long filament wires require supporting members between the ends of the filament array in order to prevent bowing of the filament wires when the tube is acted upon by mechanical and electromagnetic forces. If the tube is subjected to shock or vibration, the incandescent filament wires will be distorted out of alignment by the lateral motion imparted to the unsupported lengths of filament wire between the attached ends. The changes in space relationship between the respective filament wires will produce corresponding changes in the spacing between the filament array and the closely spaced grid. If a power tube of this type is located transversely between the poles of an external magnet, as in a magnetically beamed power tube, the current flowing in the filament wires will be perpendicular to the magnetic field between the poles of the magnet. As demonstrated by the Right Hand Rule, the interaction of the magnetic field with the current in the filament wires will produce a lateral force on the filament strands which will bow them out of their positional relationship with the opposing grid rods. Furthermore, if the filament wires are heated with an alternating current, the directional changes in current flow will co-act with the magnetic field to cause the filament wires to vibrate at the same frequency as the alternating current. This bowing back and forth of the long, unsupported lengths of filament wire will affect the electrical characteristics of the tube and possibly cause breakage of the filament wires between the attached ends. Therefore, in order to hold the filament wires in axial alignment and maintain uniform spacing between the filament array and the adjacent grid rods, supporting members should be provided to restrain lateral motion of the long filament wires between the supported ends of the filament array. However, while restraining lateral motion, these intermediate support members should permit longitudinal movement of the respective filament wires so that thermal elongation can take place during the operation of the tube.

In many power tubes of this type, alternate filament wires in the linear array are connected to one cathode terminal of the tube and the intervening filament wires are connected to the other cathode terminal. With adjacent filament strands carrying current in opposite directions, the problem of providing intermediate supporting members for the filament wires becomes complicated. However, in our co-pending patent application, Ser. No. 65,091, filed Sept. 1, 1967, and assigned to the assignee of this invention, we described a cathode structure having a linear array of parallel filament wires wherein all the filament wires connecting to one cathode terminal of the tube are arranged as a group on one side of a center mast and all the filament wires connecting to the other cathode terminal are on the opposite side of the center mast. This arrangement of filament wires has the advan tage of permitting the use of an uninsulated supportin' member for each group of filament wires. Therefore, two intermediate supporting members, one for each group of filament wires, can be disposed in planes transverse to the filament wires; and the respective filament wires can pass through respective over-sized holes in the common supporting members. Thus, the filament wires can expand longitudinally through the respective larger diameter holes; and the periphery of the holes can restrain lateral movement of the respective filament strands. However, it has been found that, if a common supporting member engages the filament wires of a group at points that are equidistant from the ends of the filament array, it does not connect points on the filament wires that are necessarily equipotential. In theory, two filament wires of equal length have the same resistance value, and with the same value of current flowing in each wire, they should have the same voltage drop at points equidistant from the ends of the wires. However, in practice, equal lengths of filament wire do not have the same value of resistance; and, with a high value of current flowing through filament wires of equal length, the differences in resistance show up as significant differences in voltage drop at points equidistant from the ends of the wires. Since the intermediate supporting members for each group of filament wires can connect points of potential difference, an equalization current can flow through the supporting members from the points of high potential to the points of low potential. These equalization currents are tolerable as long as they are steady and do not overheat the interconnecting support members.

When a tube having the described filament structure is subjected to shock or vibration, the restricted movement of the respective filament wires within the relatively larger diameter holes of the intermediate support members causes the filaments to make and break contact with the periphery of the respective holes, thereby interrupting and reestablishing the flow of equalization currents. These intermittent contacts between the filament wires and the intermediate support members produce electrical noise in the output of the tube and minute arcing between the contacting members. The arcing that occurs between the filament wires and the intermediate supporting members vaporizes material from the contacting surfaces which vaporized material redeposits elsewhere in the tube. Some of the vaporized metal will deposit on surfaces of dielectric members and degrade their insulating properties. The vaporized metal will also deposit on the electrodes of the tube in the form of whisker growths and become points of highly concentrated voltage potential. These high voltage points or the minute arcing between the filament wires and the supporting members will eventually lead to areing between the electrodes and voltage breakdown of the tube. Therefore, the problem is to restrain lateral movement while allowing longitudinal movement of the filament wires without causing the undesirable effects due to intermittent contacting surfaces.

SUMMARY OF THE INVENTION This invention provides thin metallic suporting arms having spaced apertures located in the center of respective intersecting slots, thus forming four resilient tangs where the arcuate perimeters of the respective apertures merge with the oblong perimeters of the respective slots. Respective filament wires of slightly larger diameter are drawn through the respective apertures in the metallic support arms by virtue of the adjacent tangs yielding in the direction of filament movement and forcing the respective slots to open slightly wider in the respective central portions. The adjacent edges of the respective tangs press on the contacting surfaces of the respective filament wires and sink into the softened material of the filament wires at high temperatures. When this occurs, the respective slots tend to return to their original configuration and the arcuate portions of the respective apertures close in on the respective filament wires. The respective tangs remain in a flexed condition and maintain spring pressure on the sides of the respective filament wires. This pressure restrains lateral movement of the filament wires during shock and vibration and maintains good electrical contact between the respective filament wires and the respective support arms during longitudinal movement of the filaments. The flexibility of the respective thin metallic support arms allows the respective filament wires to move longitudinally when undergoing thermal elongation at high temperatures.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a fragmentary axial sectional view of a tube embodying the invention;

FIG. 2 is an enlarged, fragmentary ViQW Q h mgans used to insulatingly support one end of the intermediate support arms;

FIG. 3 is an enlarged view of one of the filament support arms;

FIG. 4 is an enlarged, fragmentary view of one of the slotted holes in a filament supporting arm just before a filament wire passes through it;

FIG. 5 is an enlarged, fragmentary view of one of the slotted holes in a filament supporting arm as a filament wire passes through it; and

FIG. 6 is an enlarged, fragmentary view showing how the resilient tangs maintain lateral pressure on the respective filament wires.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawing, wherein like characters of reference designate like parts throughout the several views, a tube embodying the invention is shown in FIG. 1 with the grid and part of the anode removed for purposes of clarity. The tube comprises a gas-tight envelope closed at one end by a metallic anode cup 11, preferably copper. The open end of the anode cup 11 terminates in an outwardly extending flange 12 which serves as the anode terminal of the tube. One end of Kovar sleeve 13 is peripherally sealed to the open end of anode cup 11 and has an outwardly extending flange at the opposite end which is hermetically attached to a similar flange on the adjacent end of Kovar sleeve 14. The other end of Kovar sleeve 14 is circumferentially sealed to one end of a dielectric cylinder 15, preferably ceramic, which is similarly sealed at its opposite end to one end of Kovar sleeve 16. The other end of Kovar sleeve 16 terminates in an outwardly extending flange which is hermetically attached to a metallic collar, preferably copper, of annular grid terminal 17. One end of Kovar sleeve 18 is peripherally secured to the opposite side of grid terminal 17 and the other end is similarly sealed to one side of a dielectric ring 19. One end of Kovar sleeve 20 is sealed, around the entire periphery, to the other side of dielectric ring 19. The opposite end of Kovar sleeve 20 is hermetically attached to one side of annular cathode terminal 21, preferably copper. Kovar sleeve 22 is hermetically attached at one end to the other side of cathode terminal 21 and is similarly sealed at the opposite end to one side of dielectric ring 23, preferably ceramic. The other side of dielectric ring 23 is sealed to one end of Kovar sleeve 24 which, in turn, is peripherally attached at the other end to one side of annular cathode terminal 25, preferably copper. Cathode terminal 25 extends radially inward and is hermetically attached to the periphery of a metallic support disc (not shown), preferably copper. Exhaust tubulation 26 is circumferentially sealed to the periphery of a central aperture in the support disc and is sealed off after processing of the tube is completed. Four, symmetrically located metallic posts 31, preferably copper, have one end mounted to the inner surface of the support disc by conventional means, such as brazing for example, and extend longitudinally within the gas-tight enclosure toward the anode of the tube. A metallic support deck 32, preferably copper, is attached to the other ends of the support posts 31 by conventional means, such as brazing for example. Thus, circular support deck 32 is electrically connected to cathode terminal 25 through an intervening metallic support structure. Cathode terminal 21 extends radially inward and is peripherally attached to a second metallic support disc (not shown), preferably copper, that has large apertures through which metallic posts 31 insulatrngly extend in spaced relationship. Metallic connecting post 33, preferably copper, has one end attached to the center of the second metallic support disc by conventional means, such as brazing for example, and extends longitudmally along the center line of the structure toward support deck 32.

Parallel connecting rods 34, 35 and 36 extend through respective apertures in support deck 32 and into the cavity of anode cup 11. Parallel connecting rods 37, 38 and 39 extend through central apertures in respective dielectric bushings 40 which are press-fitted into holes in support deck 32. Connecting rods 37-39 also extend longitudinally into the cavity of anode cup 11. Connecting rods 34-39 are disposed in the same axial plane of the tube and form a linear array which extends through the longitudinal center line of the tube. On the supported side of deck 32, respective helical springs 41 encircle the ends of respective connecting rods 34-36 and have one end nesting in respective annular cavities in support deck 32. Respective helical springs 41 encircle the ends of respective connecting rods 37-39 and have one end nesting in respective annular cavities in the respective dielectric bushings 40. Respective cup washers 42 slidingly engage the respective connecting rods 34-39 and encircle the other ends of respective helical springs 41. One end of respective conducting straps 43-45, preferably copper, engages respective connecting rods 34-36 and abuts the annular surface of respective cup washers 42. One end of respective conducting straps 46-48, preferably copper, engages respective connecting rods 37-39 and abuts the annular surface of respective cup washers 42. Respective cap nuts 49, pressed onto the ends of the respective connecting rods 34-39, have outwardly extending annular flanges which bear against the adjacent surfaces of respective conducting straps 43-48. The other ends of respective conducting straps 43-45 are attached to support deck 32 by any convenient means, such as respective screws journalled through apertures in the respective conducting straps 43-45 and into threaded holes, not shown, in support deck 32. The other ends of respective conducting straps 46-48 are attached to the distal end of connecting post 33 by any convenient means, such as screw 50 journalled through apertures in the respective conducting straps 46-48 and into a threaded cavity in the adjacent end of connecting post 33. Slack is left in the respective conducting straps 43-48 between the attached ends to permit longitudinal movement of the respective connecting rods 34-39 during operation of the tube.

Center mast 51, having one end attached to the other side of support deck 32 by conventional means, such as brazing for example, extends longitudinally along the center line of the tube and into the cavity of anode cup 11. At a predetermined distance from support deck 32, the diameter of center mast 51 is reduced, as shown in FIG. 2, thereby forming annular shoulder 52 and shank portion 53. A cup Washer 54 encircles the shank diameter 53 of center mast 51 and abuts the annular surface of shoulder 52. The rim of cup washer 54 surrounds the shoulder flange 56 of a dielectric bushing 55 which slidingly engages the reduced diameter 53 of center mast 51. As shown more clearly in FIG. 3, a thin metallic supporting member, preferably tungsten, has a large aperture 71 at one end and three smaller apertures 72 in the center of longitudinal slots 73 located in the armlike portion of the supporting member. Slots 73 are separated and surrounded by thin metallic frames which are integral parts of the support arm. The arm portion is joined to the periphery of the large aperture 71 by an integral, narrow section 74 which increases the flexibility of the supporting member in that area. The periphery of a large aperture 71 in support arm 57 encircles the shank portion of dielectric bushing 55 and bears against the annular surface of the dielectric shoulder flange 56. The arm of support member 57 extends radially from the center mast 51 in the same axial plane as the linear array of connecting rods 34-39. The respective apertures 72 in support arm 57 are aligned with the distal ends of the respective connecting rods 37-39. A dielectric washer 58 encircles the shank diameter of bushing 55 and interfaces with the periphery of large aperture 71 in support arm 57. The periphery of large aperture 71 in support arm 59 encircles the shank portion of dielectric bushing 55 and bears against the other side of dielectric washer 58. Support arm 59, which is identical in shape with arm 57, extends radially from center mast 51, diametrically opposite to support arm 57 and in the same axial plane as the linear array of connecting rods 34-39. The respective apertures 72 in support arm 59 are aligned with the distal ends of the respective connecting rods 34-36. A dielectric washer 60 encircles the shank end of dielectric bushing and has a thickness which insures that it will protrude beyond the end of dielectric bushing 55. Dielectric washer nests within the rim of metallic cup washer 61 which slidingly engages the reduced diameter 53 of center mast 51. A metallic sleeve 62 surrounds the shank portion 53 of center mast 51 and has one end bearing against the annular surface of cup washer 61. A dielectric bushing 63 (FIG. 1) having a shoulder flange 64 slides onto the reduced diameter 53 of center mast 51 and interfaces with the other end of metallic sleeve 62. The shank end of dielectric bushing 63 engages one end of a central bore in cross arm 65 and shoulder flange 64 insulates cross arm 65 from metallic sleeve 62. Cross arm 65 extends radially from opposite sides of center mast '51 in the same axial plane as the linear array of connecting rods 34-39 and is provided with spaced apertures. that are aligned with the distal ends of the respective connecting rods 34-39. A dielectric bushing 67 having an annular shoulder flange 66 slidingly engages the reduced diameter 53 of center mast 51. The shank of dielectric bushing 67 protrudes into the other end of the central bore in cross arm 65, and the annular surface of shoulder flange 66 bears against the adjacent surface of cross arm 65. A metallic nut 69 is journalled onto the threaded end of center mast 51 and presses against the adjacent surface of dielectric bushing 66. This pressure is transmitted through interfacing surfaces of the assembled parts until the adjacent surface of cup Washer 52 is forced against annular shoulder 54 of center mast 51. Six filament wires 80, preferably tungsten, are precut to the length required. One end of the respective filament wires 80 passes through a respective aperture in cross arm 65 and is gently forced through. a respective aperture 72 in the respective support arms 57 and 59. The filament wires 80 are drawn through the apertures 72 until the other ends of the respective filament wires enter the respective apertures in cross arm 65. Filament wires 80 are aflixed to cross arm 65 by conventional means, such as welding for example. The helical springs 41 that engage the ends of the respective connecting rods 34-39 on the other side of support deck 32 are depressed and the adjacent ends of the respective filament wires 80 are inserted into cavities in the distal ends of the respective connecting rods 34-39. Filament wires-80 are attached to the connecting rods 34-39 by conventional means, such as welding for example, and the helical springs 41 are released.

As shown in FIG. 4, flexible corner tangs 75 are formed in the thin metallic support arms 57 and 59 where the arcuate portions of the respective apertures 72 merge with the oblong portions of the respective slots 73. As indicated by the dashed circle 81, the diameters of the respective filaments are slightly larger than the diameters of the respective apertures 72. When the ends of the respective filament wires 80 are urged against the peripheries of the respective apertures 72, the tangs 75 yield and cause the areas outlined by the dashed lines 76 to flex in the direction of the applied pressure. The edges 77 of the respective tangs 75 are bent in the direction of filament travel and provide sliding surfaces for the contacting surfaces of the respective filament wires 80. Continued pressure on the ends of the respective filament wires 80 is transmitted through the flexed tanks 75 to the integral material on each side of the respective slots 73, causing the areas 78, shown in FIG. 5 by the dashed lines, to flex in the direction of filament travel also. The flexing of areas 78 opens the central portions of the respective slots 73 slightly wider and allows the respective filament wires 80 to pass through. Respective small gaps 79 between the peripheries of the respective filament wires 80 and the unflexed arcuate portions of the respective apertures 72 indicate that all of the lateral pressure exerted by the respective filament wires 80 in passing through the respective apertures 72 is against the edges 77 of the respective tangs 75. It should also be noted that all of the pressure exerted on the sides of the respective filament wires 80 by the resiliency of the respective thin metallic arms 57 and 59 is exerted through the respective contacting tangs 75. When a high value of current flows through the respective filament wires 80 heating them to incandescene as during the subsequent carburizing process, the material of the filament wires softens slightly and allows the contacting edges 77 of the respective tangs 75 to penetrate into the filament wire material, as shown in FIG. 6. As a result, the flexed areas 78 adjacent the sides of the respective slots 73 begin to flatten out and become more planar, depending upon the amount of interference between the diameters of the respective filament wires 80 and the diameters of the respective apertures 72. As the areas 78 unfiex, the respective slots 73 begin to assume their original configuration and the gaps 79 to decrease in size. Thus, the unflexed arcuate portions of the respective apertures 72 close in around the peripheries of the respective filament wires 80 and may even bite into the softened material of the filament wires depending again upon the overlapping interference between the respective wires 80 and the respective apertures 73. However, the flexed areas 78 and flexed tangs 75 do not return completely to their original plane, and the respective resilient tangs 75 continue to maintain a lateral pressure on the respective filament wires 80.

When the tube is used in high power applications, the heavy current flowing through the filament wires heats the respective filaments 80 to an incandescent state. As a result, the respective filament Wiress 80 undergo thermal expansion and elongate in the longitudinal direction. The respective helical springs 41 surrounding ends of the respective connecting rods 34-39 expand in the longitudinal direction to compensate for the thermal elongation of the respective filaments 80. As a result, the respective filament wires 80 move in the longitudinal direction and the narrow sections 74 in the respective support arms 57 and 59 flex. Any differences in longitudinal expansion between the respective filament wires 80 is permitted by the respective thin metallic arms 57 and 59 flexing in the narrow metallic regions adjacent the end of the respective slots 72. Thus, the longitudinal movement of the filament wires is accomplished without any sliding taking place between the respective filament wires 80 and the surrounding material of the respective support arms 57 and 59. The respective resilient tangs 75, having penetrated into the respective filament wires 80 and exerting a pressure on the abutting surfaces, maintains electrical contact with the respective filament wires 80. Since any equalization currents flowing through the connecting support arms are not interrupted, the undesirable effects of minute arcing, such as electrical noise and metallic vaporization, are avoided. Any lateral movement of the respective filament wires 80, such as might occur during shock or vibration for example, is restrained by the lateral pressure of the respective, symmetrically disposed, tangs 75 and the closely surrounding perimeters of the respective apertures 72 in the respective support arms 57 and 59. Thus, there has been disclosed and described herein a novel filament support structure which restrains lateral movement while allowing longitudinal movement of the filament wires without producing the undesirable effects due to intermittent contact and consequent minute arcing.

As filament support members, the thin metallic arms 57 and 59 should have a narrow cross section to impede the flow of heat away from the contacting surfaces of the respective filament wires 80. However, the cross section of the support arms 57 and 59 should be large enough to conduct the flow of equalization currents without overheating. It should also be pointed out that more than one pair of intermediate supporting arms, like 57 and 59, can be located along the length of the center mast 51 to reduce the unsupported lengths of filament wire to smaller increments of the total length. Obviously, the longer the filament wires the more lateral restraint will be required to maintain axial alignment and uniform interelectrode spacing. By proper axial location of the thin metallic support arms, opposition to lateral movement of the filaments can be applied where mechanical and electromagnetic forces have the greatest effect. These and other modifications which may occur to those skilled in the art are not intended to limit the spirit and scope of this invention as set forth in the claims appended hereto.

What is claimed is:

1. An electrode structure comprising:

a fixed support;

a resilient member extending fro-m the fixed support and having an aperture in a portion thereof; and

an elongated member longitudinally disposed within the aperture and having a cross section larger than the aperture;

said resilient member further having resilient means for allowing a portion of the elongated member to pass longitudinally through the aperture, which means penetrates and maintains a lateral pressure on other portions of the elongated member.

2. An electrode structure as set forth in claim 1 wherein said resilient member is a thin metallic arm.

3. An electrode structure in a gas-tight envelope comprising:

a support fixedly mounted in the envelope;

a resilient arm having one end attached to the support and having spaced apertures therein;

a plurality of interconnected heated members disposed longitudinally in respective apertures in said arm, each heated member having a cross-section at least as large as said respective apertures; and

resilient means disposed adjacent each of said apertures for yieldably allowing portions of the respective heated members to pass longitudinally through the respective apertures, said means having portions thereof embedded in adjacent portions of the heated members and maintaining lateral pressure on said adjacent portions of the heated members.

4. A filament structure comprising:

a fixed oblong support;

a resilient member having a portion attached to the fixed support and a portion extended outwardly from the fixed support, said extended portion having spaced apertures therein;

an array of filament wires extended longitudinally through respective apertures in said resilient mem ber, each filament wire having a cross-section at least as large as the associated aperture; and

resilient tangs disposed adjacent each aperture and having respective portions attached to the resilient member and other respective portions embedded into adjacent portions of a respective filament wire.

5. A filament structure comprising:

an elongated support;

a resilient arm extending radially from the support and having spaced apertures therein;

an array of parallel filament wires passing longitudinally through respective apertures in the resilient arm, each wire having a diameter larger than that of the respective aperture; and

means for allowing portions of each filament wire to pass through a respective aperture, which means penetrates into and maintains opposing lateral pressures on the adjacent portion of the filament wire.

t5. A filament structure in a gas-tight envelope comprising:

an elongated support mounted on the envelope;

a resilient arm having one end attached to the elongated support and radially extended therefrom;

said arm having a plurality of slots spaced apart longitudinally of the resilient arm and further having apertures which intersect intermediate portions of respective slots thereby forming resilient tangs at said intersections; and

a linear array of parallel filament wires disposed parallel with the support, each wire having a portion disposed within a respective aperture and having side portions interlocked with respective adjacent tangs.

7. A filament structure in an electron tube comprising:

a linear array of parallel filament wires located longitudinally in a central axial plane of the tube;

a center mast located on the longitudinal center line of the tube;

resilient arms radially extended from opposing sides 1 of the center mast in planes perpendicular to the plane of the linear array of filament wires;

spaced apertures located in the respective resilient arms and having arcuate peripheral portions which merge with respective peripheries of intersecting radially 20 extending slots, which apertures have marginal portions engaging transverse portions of respective filament wires; and

resilient tangs located adjacent the intersection of each aperture and respective slot, said tangs having edges embedded in a respective contacted portion of a respective filament wire and thereby held in a flexed condition.

References Cited UNITED STATES PATENTS 10 3,407,328 10/1968 Kendall 313 27s 3,218,502 11/1965 Freggens 313348 2,920,226 1/1960 Bassett 313 27s JOHN HUCKERT, Primary Examiner B. ESTRIN, Assistant Examiner US. Cl. X.R. 

